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根据第2、3小节得到的中断概率与平均误码率封闭表达式,本小节分析了基于波长分集的水下无线光通信系统受到传输路径损耗和各向异性海洋湍流影响下的性能变化。由水下无线光通信系统信道传输衰减特性可知[4,32],不同海水类型对光的衰减作用有所差异,图2所示为近海水质中不同波长光信号总衰减系数[33-34],在550~590 nm之间的光波衰减系数较小,其中波长为570 nm的光信号在传输过程中受到的吸收、散射效应最小。因此,文中选取受近海水质衰减影响较小的560、570、580 nm光波信号应用于UWOC系统波长分集方案。波长分集可由分集阶
$w{\rm{ = }}1,2,\cdots,W$ 决定,无波长分集$ (W=1)$ 采用570 nm波长工作,2阶分集$ (W\rm{=}2)$ 工作波长为570 nm和560 nm,具有3阶分集$ (W=3)$ 时工作波长为570、560、580 nm。仿真中所用到的参数如表1所示。Figure 2. Total attenuation coefficient of optical signals of different wavelengths in coastal ocean
Coefficient Value Ratio of temperature and salinity contribution to ocean turbulence, $\omega $ ${\rm{ - }}1$ Kinetic energy dissipation rate, $\varepsilon /{{\rm{m}}^2} \cdot {{\rm{s}}^{{\rm{ - }}3}}$ ${10^{{\rm{ - 4}}}}$ Mean square temperature dissipation rate, ${\chi _T}/{{\rm K}^2} \cdot {{\rm{s}}^{{\rm{ - }}1}}$ ${10^{{\rm{ - 4}}}}$ Dynamic viscosity coefficient, $\nu /{{\rm{m}}^2} \cdot {{\rm{s}}^{{\rm{ - }}1}}$ ${10^{{\rm{ - 5}}}}$ Receiver diameter, $D/{\rm{mm}}$ $1$ Transmission distance, $L/{\rm{m}}$ $10$ Table 1. Simulation parameters
接收机对三种波长光信号的接收响应度不同。仿真中设置560、570、580 nm波长光信号对应的光子检测效率
$\eta $ 分别为32%、30%、29%。由公式$\gamma {\rm{ = }}\dfrac{{M\eta e}}{{hv}}$ 可得到接收机对不同光波长的响应度,其中$e$ 表示电子电量,$h$ 表示普朗克常数,$v$ 表示光频,$M$ 表示雪崩光电二极管倍增系数。结合三种波长光信号在近海水质中的衰减效应以及接收机的响应度,可以计算得到不同工作波长之间的平均信噪比关系。波长为570 nm的光信号在近海水质中传输平均信噪比为$\left\langle {SN{R_1}} \right\rangle $ ,当传输距离为10 m时,560 nm和580 nm的波长信号平均信噪比$\left\langle {SN{R_2}} \right\rangle $ 、$\left\langle {SN{R_3}} \right\rangle $ 分别为$0.95\left\langle {SN{R_1}} \right\rangle $ 、$0.{\rm{78}} $ $ \left\langle {SN{R_1}} \right\rangle $ 。当传输距离等于5 m时,560 nm和580 nm的波长光信号平均信噪比关系分别为${\rm{1}}{\rm{.02}}\left\langle {SN{R_1}} \right\rangle $ 、$0.{\rm{87}}\left\langle {SN{R_1}} \right\rangle $ ;在15 m的传输距离下,560 nm和580 nm的波长光信号平均信噪比关系分别为$0.{\rm{9}}\left\langle {SN{R_1}} \right\rangle $ 、$0.{\rm{7}}\left\langle {SN{R_1}} \right\rangle $ 。在进行波长分集UWOC系统性能仿真时,每个图中平均SNR表示采用570 nm波长光信号在无波长分集系统中传输的平均信噪比$\left\langle {SN{R_1}} \right\rangle $ 。当$W{\rm{ = 2}}$ 时,570 nm和560 nm的光信号在海水信道中传输受到的衰减效应有所差别,此时得到系统中断概率与平均误码率中包含经过两个不同衰减的平均信噪比。同样地,在$W{\rm{ = 3}}$ 的系统中,海水信道对570、560、580 nm波长光信号的衰减效应不同,传输过程中分别对应三个平均信噪比,最终得到3阶波长分集下UWOC系统性能的变化。根据公式(10)获得各向异性海洋湍流等效结构参数,图3(a)~(c)绘制了在不同各向异性因子下波长分集UWOC系统的中断概率变化,表2给出了平均信噪比30 dB时不同各向异性因子下无波长分集与2、3阶波长分集UWOC系统中断概率的具体数值。随着各向异性因子
${u_x}$ 、${u_y}$ 分别在$x$ 和$y$ 方向上同时增加,无波长分集和2、3阶波长分集的UWOC系统中断概率明显降低。比如在$W{\rm{ = }}3$ 的波长分集UWOC系统中,增加各向异性因子${u_x}$ 和${u_y}$ ,系统中断概率从$3.819 \times {10^{{\rm{ - 7}}}}$ 减小至$3.347 \times {10^{{\rm{ - 10}}}}$ 。这是因为各向异性因子增大,海洋湍流内部由于不对称性加剧导致结构密度降低,相邻涡流单元层碰撞减少,使湍流折射率变化起伏降低,海洋湍流强度减小,导致闪烁效应减弱,使UWOC系统具有了较小的中断概率。当各向异性因子相等时,相较于无波长分集UWOC系统,使用波长分集的系统中断概率更低,且$W{\rm{ = }}3$ 的波长分集UWOC系统比$W{\rm{ = 2}}$ 的UWOC系统性能更好。比如在各向异性因子${u_x} = 1,{u_y} = 2$ 时,无波长分集UWOC系统中断概率为$2.798 \times {10^{{\rm{ - 3}}}}$ ,2、3阶波长分集UWOC系统中断概率分别为$1.199 \times {10^{{\rm{ - 5}}}}$ 、$1.{\rm{749}} \times $ $ {10^{{\rm{ - 7}}}}$ 。Figure 3. Outage probability performance of wavelength diversity UWOC system under different anisotropic factors. (a) No wavelength diversity; (b) Second-order wavelength diversity; (c) Third-order wavelength diversity,
$L{\rm{ = }}10\;{\rm{m}}$ Wavelength diversity ${u_x} = 1,{u_y} = 1$ ${u_x} = 1,{u_y} = 2$ ${u_x} = 2,{u_y} = 2$ $W{\rm{ = 1}}$ $3.{\rm{792}} \times {10^{{\rm{ - 3}}}}$ $2.798 \times {10^{{\rm{ - 3}}}}$ $2.{\rm{499}} \times {10^{{\rm{ - 4}}}}$ $W{\rm{ = 2}}$ $2.097 \times {10^{{\rm{ - 5}}}}$ $1.199 \times {10^{{\rm{ - 5}}}}$ $1.{\rm{228}} \times {10^{{\rm{ - 7}}}}$ $W{\rm{ = 3}}$ $3.819 \times {10^{{\rm{ - 7}}}}$ $1.{\rm{749}} \times {10^{{\rm{ - 7}}}}$ $3.347 \times {10^{{\rm{ - 10}}}}$ Table 2. Outage probability of wavelength diversity UWOC system under different anisotropy factors
图4给出在10 m的传输距离下接收端使用OC和EGC技术的波长分集UWOC系统平均BER变化。当各向异性因子
${u_x} = 1,{u_y} = 2$ ,平均信噪比为30 dB时,2阶波长分集UWOC系统接收端使用OC和EGC的平均BER分别为$7.634 \times {10^{{\rm{ - 5}}}}$ 、$1.355 \times $ $ {10^{{\rm{ - 4}}}}$ ,3阶波长分集UWOC系统接收端使用OC和EGC的平均BER分别等于$7.375 \times {10^{{\rm{ - 6}}}}$ 、$2.707 \times {10^{{\rm{ - 5}}}}$ 。根据数值结果表明,相同阶数的波长分集UWOC系统接收端使用OC技术比EGC技术得到的平均误码率更低。Figure 4. Comparison of the average BER performance of the UWOC system without wavelength diversity using OC and EGC technology under different anisotropic factors,
$L{\rm{ = }}10\;{\rm{m}}$ 图5和图6更加详细地表明了使用波长分集的UWOC系统在不同海洋湍流参数下的性能变化。选取各向异性因子
${u_x} = 1,{u_y} = 2$ 时,不同的动能耗散率$\varepsilon $ 、均方温度耗散率${\chi _T}$ 、温度与盐度对海洋湍流贡献比值$\omega $ 对波长分集UWOC系统中断概率和平均误码率的影响。改变每个图所示变量,给定其余海洋湍流参数。由图5(a)和图6(a)可知,随着湍流中动能耗散率的增加,波长分集UWOC系统的中断概率和平均误码率逐渐降低。这是因为动能耗散率决定湍流中的能量转化,单位流体质量的动能耗散率越大,湍流中转化为分子热能的能量越快,海洋湍流强度越弱,此时系统性能受到的影响减小。从图5(b)和图6(b)中可以看出,当均方温度耗散率减小时,波长分集UWOC系统中断概率与平均误码率降低。这是因为均方温度耗散率描述湍流对流体温度场的影响,当均方温度耗散率减小时,分子热传导作用对温度的波动影响变小,系统性能受湍流影响减弱。从图5(c)和图6(c)中可以发现,波长分集UWOC系统的中断概率与平均误码率随着温度与盐度对海洋湍流贡献比值$\omega $ 的增大而增加。$\omega $ 越大,表明盐度引起的海洋湍流的贡献越大,湍流强度变大,系统的通信系统性能恶化。Figure 5. Outage probability performance of wavelength diversity UWOC system under different ocean turbulence parameters, (a) kinetic energy dissipation rate
$\varepsilon $ , (b) mean square temperature dissipation rate${\chi _T}$ , (c) the ratio of temperature and salinity contribution to ocean turbulence$\omega $ ,$L{\rm{ = }}10\;{\rm{m}}$ Figure 6. Average BER performance of wavelength diversity UWOC system under different ocean turbulence parameters, (a) kinetic energy dissipation rate
$\varepsilon $ , (b) mean square temperature dissipation rate${\chi _T}$ , (c) the ratio of temperature and salinity contribution to ocean turbulence$\omega $ ,$L{\rm{ = }}10\;{\rm{m}}$ 图7给出在近海水质和各向异性因子
${u_x} = 1, $ $ {u_y} = 2$ 的海洋湍流条件下,波长分集UWOC系统经过不同传输距离时的平均BER变化曲线,表3给出了平均信噪比30 dB时不同传输距离下无波长分集与2、3阶波长分集UWOC系统平均BER的具体数值。当传输距离从5 m增加到15 m时,无波长分集UWOC系统平均BER从$3.{\rm{42}} \times {10^{{\rm{ - 5}}}}$ 变为$4.308 \times {10^{{\rm{ - 3}}}}$ ;2阶波长分集UWOC系统平均BER从$1.481 \times {10^{{\rm{ - 8}}}}$ 变为$2.198 \times $ $ {10^{{\rm{ - 4}}}}$ ;同样地,3阶波长分集UWOC系统平均BER从$8.761 \times {10^{{\rm{ - 11}}}}$ 变为$3.604 \times {10^{{\rm{ - 5}}}}$ 。随着传输距离的增加,波长分集UWOC系统平均误码率变大。这是因为增加传输距离,不同波长光信号传输时受到的海水衰减和海洋湍流效应影响逐渐加剧,导致UWOC系统性能下降。当处于同一传输距离时,使用波长分集的UWOC系统比无波长分集系统性能更好,并且3阶波长分集比2阶波长分集的UWOC系统平均误码率明显改善。比如当传输距离等于5 m时,无波长分集UWOC系统平均BER为$3.{\rm{42}} \times {10^{{\rm{ - 5}}}}$ ,而$W{\rm{ = 2}}$ 和$W{\rm{ = 3}}$ 的波长分集UWOC系统平均BER分别为$1.481 \times $ $ {10^{{\rm{ - 8}}}}$ 、$8.761 \times {10^{{\rm{ - 11}}}}$ 。Figure 7. Average BER performance of wavelength diversity UWOC system using OC technology under different transmission distances
Wavelength diversity $L{\rm{ = }}5\;{\rm{m}}$ $L{\rm{ = 10\;m}}$ $L{\rm{ = 1}}5\;{\rm{m}}$ $W{\rm{ = 1}}$ $3.{\rm{42}} \times {10^{{\rm{ - 5}}}}$ $2.7 \times {10^{{\rm{ - 3}}}}$ $4.308 \times {10^{{\rm{ - 3}}}}$ $W{\rm{ = 2}}$ $1.481 \times {10^{{\rm{ - 8}}}}$ $7.634 \times {10^{{\rm{ - 5}}}}$ $2.198 \times {10^{{\rm{ - 4}}}}$ $W{\rm{ = 3}}$ $8.761 \times {10^{{\rm{ - 11}}}}$ $7.{\rm{375}} \times {10^{{\rm{ - 6}}}}$ $3.604 \times {10^{{\rm{ - 5}}}}$ Table 3. Average BER of wavelength diversity UWOC system using OC technology under different transmission distances
Performance analysis of wavelength diversity wireless optical communication system in ocean turbulence
doi: 10.3788/IRLA20210131
- Received Date: 2021-07-10
- Rev Recd Date: 2021-09-25
- Publish Date: 2021-12-31
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Key words:
- underwater wireless optical communication /
- ocean turbulence /
- wavelength diversity /
- Gamma-gamma distribution /
- outage probability /
- average bit error rate
Abstract: Due to seawater absorption, scattering attenuation and ocean turbulence effects, the optical signal at the receiving end of the underwater wireless optical communication (UWOC) system will flicker. The flickering signal will result in a decrease in the transmission performance of the UWOC system. Based on the Gamma-gamma distribution of the ocean turbulence channel model, according to the equivalent structural parameters represented by ocean turbulence parameters and anisotropy factors, the closed expressions of the outage probability (OP) and the average bit error rate (BER) of the wavelength diversity UWOC system were derived. With the increase of the anisotropy factor, the changes in the outage probability and the average bit error rate of UWOC system with different wavelength diversity orders were analyzed. The average bit error rate difference of the UWOC system between the optimal combining (OC) and the equal gain combining (EGC) used at the receiving end technology were compared, and the influence of different ocean turbulence parameters and transmission distances on the performance of the wavelength diversity UWOC system was simulated. The numerical results show that the ocean turbulence effect on the UWOC system gradually weakens with the increase of the anisotropy factor. The UWOC system with wavelength diversity technology has significantly improved the outage probability and the average bit error rate than the UWOC system without wavelength diversity technology.